Old enough to be a model (7)
I believe the answer is:
example
'a model' is the definition.
(I know that model can be written as example)
'old enough' is the wordplay.
'old' becomes 'ex' (prefix meaning former or onetime).
'enough' becomes 'ample' (ample means enough or plenty).
'ex'+'ample'='EXAMPLE'
'to be' is the link.
(Other definitions for example that I've seen before include "Instance - pattern" , "Thing characteristic of its kind" , "warning" , "Illustration or model" , "Something illustrating a general rule" .)